1. Field of the Invention
The present invention relates to ceramic articles, such as ceramic heaters or the like, which are preferably adapted for application in diversified apparatuses for manufacturing semiconductors and etching apparatuses, such as for plasma CVD, low pressure CVD, plasma etching, photoetching, or the like, and manufacturing processes thereof.
2. Description of the Related Art
As a heat source in apparatuses for manufacturing semiconductors, so-called "stainless heaters" have been generally used. However, these heat sources have posed a problem such that particles generate by the action of halogenous corrosive gases. Therefore, indirect heating type wafer heating apparatuses have been developed, which comprise a container to be exposed to deposition gases or the like, equipped with an infrared lamp on the outside and provided with an infrared-transparent window on an exterior wall thereof. With the heating apparatuses of this type, infrared energy is irradiated towards a heating target composed of an excellent corrosion-resistant material, such as graphite or the like, to heat a wafer placed on the upper surface of the heating target.
However, the heaters of this type have the disadvantages of increased heat losses and lengthy temperature rise times, as compared with direct heating heaters, and insufficient efficiency in heating of the target, which is induced by gradual increases in hampering of infrared transmission and in heat absorption of the infrared-transparent window, with increasing deposition of CVD membrane thereon.
In order to solve these problems, the present inventors proposed a ceramic heater comprising a high melting metallic wire embedded within a dense ceramic disc-shaped substrate, as an integral structure. This wire is in a spiral-coiled shape within the disc-shaped substrate and connected at both ends thereof with terminals. It has been found that this ceramic heater has excellent characteristics, particularly, adapted for application in the manufacture of semiconductors.
However, it also has been found that such a disc-shaped ceramic heater presents some problems, particularly, from a manufacturing point of view.
Namely, in order to manufacture a ceramic heater as described above, a high melting metallic wire or filament is wound up into a convolution, both ends of which are connected with terminals (electrodes), respectively. A ceramic powder is charged into a press-molding machine and preformed to have a certain degree of hardness. In this case, a continuous recess or groove is provided along a predetermined planar pattern on the surface of the preform. Then, the convolution is laid in the recess and further covered with the ceramic powder. Then, the ceramic powder is subjected to a uniaxial pressure molding to provide a disc-shaped molded body which is then sintered by hot-pressing.
However, when the above convolution is laid along the predetermined planar pattern on the preform, the convolution must be manually inserted into and along the recess of the preform, which is tedious and troublesome work. Moreover, during this work, positional deviation or deformation of the convolution inevitably takes place. As a consequence, the position of the resistant heating element is not fixed as designed within the sintered ceramic substrate, thereby forming thick and thin portions in heat radiation value. As a result, the temperature of the heating surface becomes uneven and heating characteristics of the heater are not stable. The uneven temperature on the heating surface of the heater, particularly, in the case of semiconductor manufacturing apparatuses, will result in an uneven thickness of semiconductor membranes which causes defective semiconductors.
Particularly, in the case where a disc-shaped ceramic heater is practically manufactured, it has been found that elimination of the unevenness of the heating surface to achieve uniform temperatures is unexpectedly difficult. For example, a heating device for semiconductor manufacturing apparatuses are required to achieve an extremely high performance of uniform heating at a setting temperature as high as 700.degree. C., 800.degree. C. or even higher over the heating surface, to thereby restrict differences of the minimal temperature and maximal temperature from the averaged temperature within about 1%, respectively, on the heating surface.
However, it has been difficult to realize such a high performance of uniform heating in ceramic heaters having a structure as described above. Because, the convoluted resistant heating elements are usually formed from a spiral coil of a thin resistance wire, which are three-dimensionally deformable rather easily. Therefore, when the convolution is laid within a ceramic shaped body, the position of the convolution may deviate and, further, during firing the shaped body, the ceramic powder may flow, thereby moving the convolution aside resulting in deformation. However, in practical manufacturing processes, these various causes have scarcely been elucidated, so that ceramic heaters with a high performance of uniform heating as described above have been difficult to manufacture consistently.
Furthermore, a new problem has arisen in temperature control of such disc-shaped ceramic heaters. Namely, when a resistant heating element is embedded in a ceramic substrate of ceramic heaters, a convolution of resistant heating element with a constant distance between adjacent loops of the convolution has been used to provide a uniform surface heating to the ceramic heaters for heating wafers.
However, such a convoluted resistant heating element with a constant distance between adjacent loops thereof forms inevitably a marginal portion absent from resistant heating element in the periphery of the heater, whereby a ratio of the surface area of the substrate, that is, the surface area that the heat escapes from, to the heat radiation value of the embedded resistant heating element is increased. Therefore, this portion results in a cool spot on the semiconductor wafer heating surface, resulting in non-uniform heating of the semiconductor wafers.
FIG. 1 is a plan view showing an embodiment of such a ceramic heater, wherein a part of the substrate is omitted, for convenience' sake, particularly in order to display a planar layout pattern of an embedded resistant heating element. In FIG. 1, a disc-shaped heating apparatus 1 has a structure comprising a dense, gastight inorganic substrate 2 composed of silicon nitride or the like and a resistant heating element 3 of a tungsten-based metal or the like embedded in a planar convolute form within the substrate. To the resistant heating element 3, electric power from an outside source can be supplied via a lead wire (not shown), a terminal 4 at the central end and another terminal 5 at the peripheral end. Thus, the disc-shaped heating apparatus 1 can radiate heat.
However, in such a structure comprising a convolution with a constant distance between adjacent loops thereof, a cool spot is necessarily formed in the peripheral zone "C" shown in FIG. 1.
Additionally, relating to the planar pattern or layout of the embedded resistant heating element, the following problem has also been presented. Namely, ceramic heaters to be used for semiconductor wafer heating apparatuses, as shown in the embodiment thereof shown in FIG. 2, that is, a schematic cross-sectional view, have a structure comprising a dense, gastight inorganic substrate 6 composed of silicon nitride or the like and a resistant heating element 8 of a metallic material, such as tungsten, molybdenum or the like, embedded in a planar convolute form within the substrate. The central and peripheral ends of the resistant heating element 8 are connected with terminals 9, 10 for power supply, respectively, and, if required, a pit 11 for setting a thermocouple for temperature measurement and/or a conduit 7 for flowing gas therethrough from the back of the heater are provided at predetermined positions.
The ceramic heaters having the above-described structure have been manufactured according to a process as explained below. A high melting metallic wire or filament is wound up into a convolution and terminals (electrodes) are bonded to both the ends of the filament, respectively. A ceramic powder is charged into a press-molding machine and preformed to provide a certain degree of hardness. A continuous recess or groove is then formed along a predetermined planar pattern on the surface of the preform. Then, the convolution is laid in the recess or groove and further covered with the ceramic powder. Then, the ceramic powder is subjected to a uniaxial pressure molding to provide a disc-shaped molded body which is then sintered by hot-pressing. Then, a pit 11 for setting a thermocouple for temperature measurement and/or a conduit 7 for flowing gas therethrough from the back of the heater are bored, for example, by a mechanical means, at a predetermined spot of the obtained fired body or substrate 6, avoiding the embedded resistant heating element.
However, according to the above-described process for manufacturing ceramic heaters, when the pit 11 and/or conduit 7 are provided by machining, the location of the resistant heating element 8 arranged at the time of shaping had been marked in advance, and then the machining was conducted so as to avoid the above marked location. However, since the resistant heating element 8 deviates from the location marked at the time of shaping, by virtue of firing-contraction occurring during the firing and deformation of the heating element occurring during the pressure molding, the resistant heating element 8 is frequently broken during the machining, due to the above-described deviation, even when the pits or conduits are bored at boring spots predetermined so as to avoid the positions of the resistant heating element 8 indicated by the above described marks. Accordingly, the above method yields many defective products since the convolutions are readily three-dimensionally deformable as described above, whereby shifting of embedding positions occurs frequently.
Furthermore, the above-described ceramic heaters are required to have an improved performance of uniform heating over its wafer heating surface. When the resistant heating element 8 is a convoluted wire, it is necessary to decrease the distance between adjacent loops of the embedded, convoluted resistant heating element 8 to thereby prevent formation of a cool spot at the peripheral portion, in order to achieve a uniform temperature over the wafer heating surface. In this instance, the adjacent loops of the resistant heating element 8 may inadvertently contact each other causing short-circuits, thereby yielding many defective products which do not adequately perform. In addition, when the resistant heating element 8 has a round-coiled form, it is necessary to increase the thickness of the ceramic substrate 6 which is used for heating wafers. However, this poses a problem of increasing heat capacity, resulting in a dull response by disturbance in temperature control and making production of compact heaters impossible.